net: dsa: move bulk of devlink code to devlink.{c,h}

dsa.c and dsa2.c are bloated with too much off-topic code. Identify all
code related to devlink and move it to a new devlink.c file.

Steer clear of the dsa_priv.h dumping ground antipattern and create a
dedicated devlink.h for it, which will be included only by the C files
which need it. Usage of dsa_priv.h will be minimized in later patches.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>